Underground drilling, such as gas, oil, or geothermal drilling, generally involves drilling a bore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface to the bottom of the bore.
The drill bit is rotated so that it advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string from the surface. Piston-operated pumps on the surface pump high-pressure fluid, referred to as “drilling mud,” through an internal passage in the drill string and out through the drill bit. The drilling mud lubricates the drill bit, and flushes cuttings from the path of the drill bit. In the case of motor drilling, the flowing mud also powers a drilling motor, commonly referred to as a “mud motor,” which turns the bit, whether or not the drill string is rotating. The mud motor is equipped with a rotor that generates a torque in response to the passage of the drilling mud therethrough. The rotor is coupled to the drill bit so that the torque is transferred to the drill bit, causing the drill bit to rotate. The drilling mud then flows to the surface through an annular passage formed between the drill string and the surface of the bore.
A drill string may experience various types of vibration. “Axial vibration” refers to vibration in the direction along the drill string axis. “Lateral vibration” refers to vibration perpendicular to the drill string axis. Two sources of lateral vibration are “forward” and “backward,” or “reverse,” whirl. Torsional vibration is also of concern in underground drilling, and is usually the result of what is referred to as “stick-slip.” Stick-slip occurs when the drill bit, or lower section of the drill string, momentarily stops rotating (i.e., “sticks”) while the drill string above continues to rotate, thereby causing the drill string to “wind up,” after which the stuck element “slips” and rotates again. Often, the bit will over-speed as the drill string unwinds. Another possible outcome is the when the slip ends, a rebound motion will cause part of the drill string to rotate counterclockwise, which may cause one or more of the threaded joints between the drill string sections to uncouple.
Systems currently on the market, such as APS Technology's Vibration Memory Module™, determine torsional vibration due to stick-slip by measuring and recording the maximum and minimum instantaneous rotations per minute (“RPM”) over a given period of time, such as every four seconds, based on the output of the magnetometers. The amplitude of torsional vibration due to stick-slip is then determined by determining the difference between and maximum and minimum instantaneous rotary speeds of the drill string over the given period of time. Preferably, root-mean-square and peak values for the axial, lateral and torsional vibrations are recorded at predetermined intervals, such as every four seconds. The amplitudes of the axial, lateral and torsional vibration may be transmitted to the surface, e.g., via mud pulse telemetry, or stored downhole for subsequent analyses.
Unfortunately, although the existence of harmful torsional vibration, and in particular “stick-slip”, can be detected, there is currently no effective method for damping such vibration. Consequently, a need exists for an apparatus and method for damping vibration in a drill string, especially torsion vibration due to stick-slip.